721 is a centered hexagonal number (A003215).
721 is the smallest number that can be written as the difference of two positive cubes in two ways (A014440).
721 is the sum of nine consecutive primes: 721 = 61 + 67 + 71 + 73 + 79 + 83 + 89 + 97 + 101.
721 is the sum of the factorials of the digits of 16 (A061602): 721 = 6! + 1!
Source: What's Special About This Number?
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